Romance in the sims is currently down to just flirting enough, making it more detailed would be good. I think the growth and decay of the romance bar could be affected by attraction. I think relationships should have some kind of 'situation' that gives them context, like 'amazing', 'happy', 'stable', 'tense' or 'rough', that kind of thing, so we have different interactions and needs depending on the situation, to make it more alive I guess and make it more meaningful to spend time on it. Also, it would be cool to have that 'new romance' period at the beginning, giving a boost that makes it easier - but also one that doesn't last, so could be good or bad...
A good deal of the ideas about attraction and relationships could also be added to non-romantic relationships.